Re: [PATCH v7 00/15] Optimizing iommu_[map/unmap] performance

[Lu Baolu]

On 7/14/21 10:24 PM, Georgi Djakov wrote:
> On 16.06.21 16:38, Georgi Djakov wrote:
> > When unmapping a buffer from an IOMMU domain, the IOMMU framework unmaps
> > the buffer at a granule of the largest page size that is supported by
> > the IOMMU hardware and fits within the buffer. For every block that
> > is unmapped, the IOMMU framework will call into the IOMMU driver, and
> > then the io-pgtable framework to walk the page tables to find the entry
> > that corresponds to the IOVA, and then unmaps the entry.
> >
> > This can be suboptimal in scenarios where a buffer or a piece of a
> > buffer can be split into several contiguous page blocks of the same size.
> > For example, consider an IOMMU that supports 4 KB page blocks, 2 MB page
> > blocks, and 1 GB page blocks, and a buffer that is 4 MB in size is being
> > unmapped at IOVA 0. The current call-flow will result in 4 indirect 
> > calls,
> > and 2 page table walks, to unmap 2 entries that are next to each other in
> > the page-tables, when both entries could have been unmapped in one shot
> > by clearing both page table entries in the same call.
> >
> > The same optimization is applicable to mapping buffers as well, so
> > these patches implement a set of callbacks called unmap_pages and
> > map_pages to the io-pgtable code and IOMMU drivers which unmaps or maps
> > an IOVA range that consists of a number of pages of the same
> > page size that is supported by the IOMMU hardware, and allows for
> > manipulating multiple page table entries in the same set of indirect
> > calls. The reason for introducing these callbacks is to give other IOMMU
> > drivers/io-pgtable formats time to change to using the new callbacks, so
> > that the transition to using this approach can be done piecemeal.
>
> Hi Will,
>
> Did you get a chance to look at this patchset? Most patches are already
> acked/reviewed and all still applies clean on rc1.

I also have the ops->[un]map_pages implementation for the Intel IOMMU
driver. I will post them once the iommu/core part get applied.

Best regards,
baolu